Outline

Introduction:The auditory midbrain implant (AMI), designed for electrical stimulation of the inferior colliculus central nucleus (ICC) for hearing restoration, is now in clinical trials. However, since the ICC is a complex 3-D structure, we investigated in an animal model if further improvements may be achieved by stimulating across multiple regions of the ICC.

Methods: Two single shank arrays were inserted along the tonotopic gradient of the ICC in guinea pigs. We then recorded auditory cortical responses to electrical stimulation of two ICC sites within the same frequency lamina and with varying inter-site delays and levels. Finally we compared the cortical responses with multi-site stimulation (MSS) to single site stimulation (SSS) to assess which parameters provide better frequency tuning and safety per given cortical response.

Results: Frequency specificity was maintained with MSS in both cortical evoked potentials and spike activity. For any given spike rate MSS was seen to require less stimulation current per site. Placement effects were also observed, where stimulating a rostral site prior to a caudal site elicited a non-linear cortical response (and vice-versa). Finally, the effect of inter-stimulus-delay (ISD) was seen to create either facilitation or inhibition of the cortical response, depending on the relative delay used.

Conclusion: Paradigms with MSS provide benefits over SSS. They provide more specific responses (less spread) with more safety for a given cortical response. Possibly ISDs can be used to elicit variable percepts by controlling the enhancement/suppression interactions observed. Finally, MSS also permits interleaved stimulation where an overall stimulation rate can be increased while minimizing local adaptation.